1. Field
The present disclosure relates generally to inspecting structures and, in particular, to inspecting countersinks in structures. Still more particularly, the present disclosure relates to a method and apparatus for identifying inconsistencies in countersinks formed in a composite structure.
2. Background
Aircraft are being designed and manufactured with greater and greater percentages of composite materials. Composite materials are used in aircraft to decrease the weight of the aircraft. This decreased weight improves performance features such as payload capacities and fuel efficiencies. Further, composite materials provide longer service life for various components in an aircraft.
Composite materials are tough, light-weight materials created by combining two or more functional components. For example, a composite material may include reinforcing fibers bound in polymer resin matrix. The fibers may be unidirectional or may take the form of a woven cloth or fabric. The fibers and resins are arranged and cured to form a composite material.
Further, using composite materials to create aerospace composite structures potentially allows for portions of an aircraft to be manufactured in larger pieces or sections. For example, a fuselage in an aircraft may be created in cylindrical sections and then assembled to form the fuselage of the aircraft. Other examples include, without limitation, wing sections joined to form a wing or stabilizer sections joined to form a stabilizer.
In manufacturing composite structures, layers of composite material are typically laid up on a tool. The layers may be comprised of fibers in sheets. These sheets may take the form of fabrics, tape, tows, or other suitable forms. In some cases, resin may be infused or pre-impregnated into the sheets. These types of sheets are commonly referred to as prepreg.
The different layers of prepreg may be laid up in different orientations and different numbers of layers may be used depending on the thickness of the composite structure being manufactured. These layers may be laid up by hand or using automated lamination equipment such as a tape laminating machine or a fiber placement system.
After the different layers of composite material have been laid up on the tool, the layers of composite material may be consolidated and cured upon exposure to temperature and pressure, thus forming the final composite structure.
Thereafter, the composite structure may be inspected using a nondestructive inspection system to determine whether inconsistencies are present. These types of nondestructive inspection systems also may be used to inspect the composite structure after the composite structure has been processed for assembly. For example, an inspection may be performed after holes are formed in the composite structure. The formation of holes and other features such as countersinks may result in the formation of inconsistencies in the composite structure.
If an inconsistency is identified, the composite structure may be reworked. In some cases, the inconsistency may result in the composite structure being discarded, requiring a new composite structure to be manufactured. Examples of inconsistencies that may be present in a composite structure include voids, porosity, delamination, foreign object debris, and other types of inconsistencies.
Nondestructive inspection involves different analysis techniques used to evaluate the properties of an object without causing damage to the object. Nondestructive inspection may be performed in a number of different ways.
For example, nondestructive inspection may include ultrasonic testing. Ultrasonic testing involves using sound waves to inspect objects. The object tested may be comprised of different types of materials. For example, the materials may be one of steel, metals, alloys, concrete, wood, composite materials, and other types of materials.
With ultrasonic inspection, ultrasonic transducers may be used to perform the ultrasonic inspection of the structure. Ultrasonic transducers cause sound signals to travel through a structure, such as a composite structure. The responses detected from the sound signals may be used to identify a location of an inconsistency. However, this type of inspection may not provide as much information about the inconsistency as desired.
For example, the depth of an inconsistency may not be as easily detectable beyond the first delamination encountered by the sound signals. Responses to ultrasonic sound signals encountering these inconsistencies near holes, countersinks, and other features may not provide information that is as accurate as desired with currently used pulse echo ultrasonic inspection techniques.
For example, the depth of an inconsistency in a countersink, such as a delamination, formed in the surface of an object may not be as easily detectable beyond the first inconsistency encountered by the ultrasonic sound signals. A shadowing effect of a delaminated layer located over other delaminated layers in the inconsistency within the countersink is present in a pulse echo ultrasonic inspection of the countersink. The ultrasonic sound signals are reflected at the first delaminated layer in the inconsistency. As a result, other delaminated layers below the first delaminated layer encountered on the countersink may not be identifiable from the response to the sound signals.
Therefore, it would desirable to have a method and apparatus that takes into account at least some of the issues discussed above as well as possibly other issues.